Vic Seasonal Rainfall Outlook: probabilities for February to April 2007,
issued 23rd January 2007

Increased late summer to mid-autumn rains in western Victoria

The outlook for total February to April rainfall,
shows a moderate shift in the odds towards above average falls in western
Victoria and also along parts of the central coast.
The pattern of seasonal rainfall odds across Victoria is mainly a result of
higher than average temperatures in the Pacific Ocean (because of El Niño).

The chances of exceeding the median rainfall for the February to April period, are
between 60 and 65% over the western quarter of Victoria and also extending along a
narrow coastal strip towards Wilsons Promontory (see map).
So in years with ocean patterns like the current, about six
February to April periods out of ten are expected to be wetter than average in
these parts of the State, with about four out of ten being drier. However, the
outlook system has low skill at this time of the year, so this outlook should be
used with caution.

Outlook confidence is related to how consistently the Pacific and
Indian Oceans affect Australian rainfall. During the February to April period,
history shows the effect to be mostly weakly consistent over Victoria (see background information).

December was the second successive month when the Southern Oscillation Index (SOI)
was only slightly negative, with a monthly value of −3 following the
−1 in November. A link to 30-day SOI values is available in the ENSO Wrap-Up
(see next paragraph).

The neutral SOI is one of a few indicators which show that the current Pacific
El Niño pattern is beginning to weaken.
For routine updates and comprehensive discussion on the latest data relating to
El Niño, together with details on what the phenomenon is and how it
has affected Australia in the past, please see the ENSO Wrap-Up.

Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest forecast map is displayed.

More information on this outlook is available from
9.00am to 5.00pm (EST) Monday to Friday by
contacting the Climate and Consultancy section in
the Bureau's Victorian Regional Office: (03) 9669 4949.

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 22nd FEBRUARY 2007

Background Information

The Bureau's seasonal outlooks are general statements about the probability
or risk of wetter or drier than average weather over a three-month period.
The outlooks are based on the statistics of chance (the odds)
taken from Australian rainfall/temperatures
and sea surface temperature records for the tropical Pacific and Indian Oceans.
They are not, however, categorical predictions about future rainfall,
and they are not about rainfall within individual months
of the three-month outlook period.
The temperature outlooks are for the average maximum and minimum temperatures
for the entire three-month outlook period.
Information about whether individual days or weeks may be
unusually hot or cold, is unavailable.

Probability outlooks should not be used as if they were categorical forecasts.
More on probabilities is contained in the booklet
The Seasonal Climate Outlook - What it is and how to use it,
available from the National Climate Centre.
These outlooks should be used as a tool in risk management
and decision making.
The benefits accrue from long-term use, say over 10 years.
At any given time, the probabilities may seem inaccurate,
but taken over several years, the advantages of taking account
of the risks should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.

Model Consistency and Outlook Confidence:
Strong consistency means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median). In this
situation relatively high confidence can be placed in the
outlook probabilities.
Low consistency means the historical relationship, and
therefore outlook confidence, is weak.
In the places and seasons where the outlooks are most skilful,
the category of the eventual outcome (above or below median)
is consistent with the category favoured in the outlook
about 75% of the time.
In the least skilful areas,
the outlooks perform no better than random chance or guessing.
The rainfall outlooks perform best in eastern and
northern Australia between July and January,
but are less useful in autumn and in the west of the continent.
The skill at predicting seasonal maximum temperature peaks in
early winter and drops off marginally during the second half of the year.
The lowest point in skill occurs in early autumn.
The skill at predicting seasonal minimum temperature peaks in
late autumn and again in mid-spring.
There are also two distinct periods when the skill is lowest
- namely late summer and mid-winter.
However, it must always be remembered that the outlooks
are statements of chance or risk.
For example, if you were told there was a 50:50 chance
of a horse winning a race but it ran second,
the original assessment of a 50:50 chance could still have been correct.

The Southern Oscillation Index (SOI) is calculated using the
barometric pressure difference between Tahiti and Darwin.
The SOI is one indicator of the stage of El Niño or La Niña
events in the tropical Pacific Ocean.
It is best considered in conjunction with sea-surface temperatures,
which form the basis of the outlooks.
A moderate to strongly negative SOI (persistently below 10) is
usually characteristic of El Niño, which is often associated with below
average rainfall over eastern Australia, and a weaker than normal monsoon
in the north. A moderate to strongly positive SOI (persistently above +10)
is usually characteristic of La Niña, which is often associated with
above average rainfall over parts of tropical and eastern Australia,
and an earlier than normal start to the northern monsoon season.
The Australian impacts of 23 El Niño events since 1900 are summarized
on the Bureau's web site
(www.bom.gov.au/climate/enso/).